3GPP Long Term Evolution (LTE) is the fourth-generation mobile communication technologies standard developed within the 3rd Generation Partnership Project (3GPP) to improve the Universal Mobile Telecommunication System (UMTS) standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS and Evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. In an UTRAN and an E-UTRAN, a User Equipment (UE) is wirelessly connected to a radio base station commonly referred to as a NodeB in UMTS, and as an evolved NodeB (eNodeB) in LTE. A radio base station is a general term for a radio network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE. The eNodeB is a logical node in LTE and the radio base station is a typical example of a physical implementation of an eNodeB. A UE may more generally be referred to as a user device/terminal, a wireless device/terminal, or a mobile device/terminal.
FIG. 1 illustrates a radio access network in an LTE system. An eNodeB 110a serves a UE 150a located within the eNodeB's geographical area of service or the cell 100a. The eNodeB 110a is also connected via an X2 interface to a neighboring eNodeB 110b serving another UE 150b in cell 100b. 
LTE systems can be configured for both time division duplex (TDD) operation and frequency division duplex (FDD) operation. In TDD systems, the base stations transmit and receive on the same carrier frequency. Uplink (UL) and downlink (DL) transmissions are separated in time by designating subframes as either UL subframes or DL subframes. In FDD systems, separate carrier frequencies are used for UL and DL transmissions.
In both TDD and FDD systems, there is always co-channel interference that needs to be taken into account. For DL communications, a UE receives interfering signals from the base stations in neighboring cells when receiving a DL transmission from its serving cell. The neighbor cell interference impairs the reception of the desired DL signals from the serving cell. For UL communications, a serving base station experiences interference from UEs transmitting on the UL in neighboring cells.
In FDD, UL and DL are on different carrier frequencies so there is no co-channel interference between UL and DL. However, there will still be cross-channel interference that needs to be handled by duplex filters. In TDD, UL and DL are on the same frequency, so a DL transmission in one cell may cause interference with an UL transmission in a neighboring cell. To mitigate this type of interference, TDD systems are usually time synchronized and aligned such that all cells transmit and receive at the same time. Guard periods are also inserted at the switching points.
In the current LTE standard, LTE-Rel 8, seven different TDD configurations are defined for LTE-TDD systems as shown in FIG. 2. One of the prime benefits of LTE-TDD systems is that the system available bandwidth can be adjusted dynamically to the traffic patterns at the base station. In FDD systems, the bandwidth in the UL and DL are fixed and cannot be changed based on traffic patterns or on the bandwidth requirement at any node. Reconfigurable TDD systems use the same TDD frame structures as the ones described in LTE-Rel 8 and illustrated in FIG. 2, but allow the TDD configuration to be changed depending on current traffic demands. The TDD configuration may be changed, for example, using Radio Resource Control (RRC) or Medium Access Control (MAC) signaling to indicate a different TDD configuration, overriding the one indicated in a System Information Block (SIB). This signaling will most likely be based on dedicated signaling, but broadcast solutions may also be considered. The main benefit of this approach is that everything on the physical layer can be kept unchanged with very limited modifications. The signaling overhead will likely be dependent on how often switching between TDD configurations is done and how many users that require the signaling. In any case, the additional signaling is expected to be low.
In reconfigurable TDD systems, the TDD configuration may be changed depending on traffic demands on a cell-by-cell basis. As a result, two neighboring base stations may use respective different resource allocations for UL and DL, which may result in DL-to-UL interference, i.e., transmitting base station to receiving base station interference. DL-to-UL interference, referred to herein as cross-link interference, occurs when one base station, referred to herein as an aggressor base station, is transmitting on the DL, while a second base station, referred to herein as a victim base station, is receiving transmissions from a UE in the UL. The different resource allocations result in interference between the base stations, as illustrated in FIG. 1. The aggressor base station 110a transmits a signal 112 in DL to a UE 150a in a cell. Another UE 150b in a neighboring cell served by the victim base station 110b transmits a signal 151 in the UL to the victim base station 110b. The victim base station 110b will when it receives the signal 151 from the UE 150b in the serving cell also receive the interfering signal 112 from the aggressor base station 110a. 
Out of all the interference scenarios in TDD systems, DL-to-UL interference—also referred to as cross-link interference—is expected to impact the victim cell the most. This is due to that the coupling between the base stations is very high in many cases, e.g. due to a direct line-of-sight (LOS) path between them. This coupling will cause severe interference for the victim base station when receiving UL control and data channels. This will in turn affect the DL transmissions in the victim cell.
One area of particular concern is continuity of the Hybrid Automatic Repeat Request (HARQ) processes. A HARQ process is defined as the determination of HARQ responses at the DL reception in a certain DL subframe and the corresponding actual transmission of the HARQ responses in UL in another subframe. The HARQ transmission in the UL subframe takes place at least 4 ms after the reception of the DL subframe for which the HARQ responses were determined. What UL subframe that the HARQ responses such as acknowledgements/non-acknowledgements (ACK/NACK) are transmitted in, depends on the TDD configuration used in the serving cell in LTE-Rel 8 TDD. There is thus a specific mapping, or indexing, for HARQ acknowledgements related to each TDD configuration. The UL control information is transmitted from the UE to the base station via the Physical UL Control Channel (PUCCH) when no UL data is scheduled and via the Physical UL Shared Channel (PUSCH) when UL data is scheduled.
For a serving cell configured with TDD configuration 1 (as illustrated in FIG. 2), the HARQ transmission mapping will be done as explained in FIG. 4, illustrating the indexing between HARQ processes and subframes. DL subframes, 41 and 44, are illustrated with an arrow pointing downwards in the figure and UL subframes 42 are illustrated with an arrow pointing upwards. The HARQ transmission corresponding to data received in one DL subframe 41 is mapped to a certain UL subframe 42. The mapping is illustrated by a curved arrow 43, where the arrow 43 points towards the subframe to which the HARQ transmission is mapped. The HARQ transmission for two different DL subframes, 41 and 44, may be mapped to a same UL subframe 42.
Indexing between HARQ processes and subframes are not well defined when a serving cell switches between different DL/UL configurations, and even the number of HARQ processes may change. A fixed or a scheduled interruption may be needed to re-set all HARQ processes.
Furthermore, the impact of cross-link interference on HARQ acknowledgements is unknown. One problem with reconfigurable TDD systems is that when using the HARQ timing or mapping as described above, HARQ acknowledgements in one cell may fall on UL subframes experiencing significant cross-link interference from base stations transmitting in the DL at the same time in neighboring cells.